This invention relates to a device for sensing a property in the environment, such as moisture, and more particularly to a new structure for such a device.
One type of device for sensing an environmental property includes a layer of dielectric material and a pair of plates in contact with opposite faces of the layer. A circuit or instrument connected to the electrodes enables changes in an electrical characteristic of the dielectric layer caused by a change in an environmental property to be sensed. For example, the capacitance measured between the electrodes may vary with environmental humidity if the dielectric material is one that absorbs moisture from the atmosphere. U.S. patent application Ser. No. 746,809 filed June 20, 1985, and now U.S. Pat. No. 4,662,220 issued May 5, 1987, describes a sensor of tyis type. In another example, variations in the resistivity of the material is measured to determine humidity. See U.S. Pat. No. 4,083,765.
One problem encountered in constructing either type of sensor is in assuring uniform surface contact between each electrode and a surface of the dielectric layer, especially in a simple construction designed to leave the dielectric layer well exposed to the environment, as in the aforesaid U.S. Pat. No. 4,083,765 which utilizes a Nafion tube to absorb moisture and two spiral wound coils as electrodes, one against the inside and the other against the outside of the Nafion tube. If a gap develops between one of the electrodes and the dielectric material due to changes in temperature, aging, etc., the gap will affect the output of the sensor. A simple construction of dielectric and electrodes which assures continual constant surface contact, especially while providing large area exposure of the dielectric to the environment, would be of considerable value.